1. Field of the Invention
The present invention relates to the trimming of circuit elements and, more particularly, to a method of batch trimming circuit elements.
2. Description of the Related Art
Many high-precision circuits require that elements within the circuits be trimmed before the circuits can output a precise result. One of the circuit elements that is most commonly trimmed is a resistor. Resistors are trimmed to have a specific resistance, or to have a resistance that matches the resistance of another resistor. One of the most commonly used approaches for trimming a resistor is laser trimming.
FIGS. 1A-1C show views that illustrate an example of a conventional method of laser trimming. FIG. 1A shows a plan view that illustrates an example of a resistor 100 which has not been laser trimmed. FIG. 1B shows a plan view that illustrates an example of resistor 100 after one round of laser trimming. FIG. 1C shows a plan view that illustrates an example of resistor 100 after multiple rounds of laser trimming.
The method begins by applying known input conditions to a trimmable circuit, measuring the actual voltage that is output from the trimmable circuit, and then comparing the actual voltage with the predicted voltage which should be output from the trimmable circuit in response to the known input conditions.
As shown in FIG. 1A, if the actual output voltage is the same as or within an error tolerance of the predicted voltage, no laser trimming is performed. On the other hand, if the actual output voltage is outside of the error tolerance, the resistance of resistor 100 is increased by removing a portion of resistor 100, known as a mouse bite, with a laser as shown in FIG. 1B.
Following this, the voltage output from the trimmable circuit is again measured. If the actual output voltage is the same as or within the error tolerance of the predicted voltage, then the method is complete. On the other hand, if the actual output voltage remains outside of the error tolerance, then the resistance is further increased by removing another portion of the resistor with the laser, followed by another measurement of the voltage output by the trimmable circuit. As shown in FIG. 1C, several rounds of laser trimming may be necessary before the actual output voltage is the same as or within the error tolerance of the predicted voltage.
Although the process of laser trimming works adequately, one of the drawbacks is that it is quite expensive to laser trim a wafer of trimmable circuits, such as a wafer of op amp circuits. If there are 30,000 integrated circuits on a wafer and each integrated circuit needs to be laser trimmed multiple times, then a significant amount of time is required. As a result, laser trimming can have a very low throughput.
Thus, there is a need for a method of batch trimming circuit elements.